Simple Harmonic Motion of a mass hanger

1. The problem statement, all variables and given/known data
A 50 g mass hanger hangs moitionless from a partially stretched spring. When a 91 gram mass is added to the hanger, the spring stretch increases by 7 cm. What is the spring constant of the spring (in N/m)? (Assume g = 9.79 m/s2.)

3. The attempt at a solution
I tried to use the three formulas above to solve for k, however; since I don't know the equilibrium position, I can't find y, which is the displacement of the mass from the equilibrium position.